The present invention relates to optical amplifier systems and in particular to solid state and gas optical amplifier systems in which a light beam to be amplified is passed repeatedly through an amplifier medium in order to improve the gain of the amplifier system.
An optical amplifier is a basic component of a laser system and is used in almost every aspect of laser technology. Since lasers were first developed, optical amplifiers have been used to increase the output energies from laser oscillators. However, it is generally difficult to extract energy efficiently from an optical amplifier and efficient energy extraction usually requires the input of high energy light beams, so that the level of net gain is modest. This is a particular problem in amplifier systems which amplify a continuous wave light source.
In order to achieve both high gain and efficient energy extraction, cascades of progressively larger amplifiers have been used. However, this results in relatively expensive multi-component amplifier systems which can have complicated optical geometries in which it can be difficult to accurately align optical components making up the system.
An alternative way of improving gain and energy extraction is to pass a light beam two or more times through the amplifier medium of an amplifier system, optionally using non-linear phase conjugation to compensate for aberrations such as thermal distortions caused by pumping of the medium. A first example has been developed and is described by N. F. Andreev, S. V. Kuznetsov, O. V. Palashov, G. A. Pasmanik and E. A. Khazanov in xe2x80x9cFour-pass YAG:Nd laser amplifier with compensation for aberration and polarisation distortions of the wavefrontxe2x80x9d in Soviet Journal of Quantum Electronics, Volume 22, pages 800-802 (1992). This document describes a system in which a beam is passed through an amplifier four times by using polarisation multiplexing and phase conjugation. This is the maximum number of passes that can be achieved by this approach because they exploit the two orthogonal polarisations and the two possible directions of collinear propagation and they separate beams by means of polarisers. A second example of such a system has been developed and is discussed in a paper by C. B. Dane, L. E. Zapata, W. A. Newman, M. A. Norton, L. A. Hackel entitled xe2x80x9cDesign and Operation of a 150 W Near Diffraction Limited Laser amplifier with SBS Wavefront Correction, published in IEEE Journal of Quantum Electronics Vol. 31, p148, January 1995. According to this system a light beam is repeatedly directed into an amplifier medium successively at different angles. The problem with this approach is that the optical geometry of the amplifier system is quite complicated and that the system is specific to pulsed laser systems and to a particular class of slab amplifier which can be used in a 9- and 11-bounce mode. Also, this type of amplifier system requires careful alignment of the optical components which make up the system.
WO95/22187 discloses an amplifier system in which a light beam is passed through an amplifier medium four times and which compensates for depolarisation of the light beam in the amplifier medium. The light from the amplifier medium is rotated in phase through 90xc2x0 before being passed back into the amplifier medium. A relay imaging telescope is used in the arrangement for applying the phase rotation, to image the light beam from the amplifier through a polarisation rotator and onto a reflector and then back onto the amplifier medium. Oppositely directed beams overlap at all points along the path until they are separated by their polarisation which limits the number of passes which can be made through the amplifier medium without causing the system to oscillate. This limits the gain of the amplifier system disclosed in WO95/22187.
The present invention seeks to overcome some of the problems discussed above by providing a high gain multi-pass laser amplifier system in which a light beam can be repeatedly passed through the amplifier medium of the system using a simple geometry in which it is relatively easy to align the optical components of the system in order to achieve a high gain and efficient energy extraction. The present invention also seeks to provide an optical amplifier system which is generic in that it can be applied to a large number of amplifier mediums. The present invention also seeks to provide a multi-pass amplifier suitable for continuous wave (cw) applications as well as for pulsed operation.
According to the present invention there is provided a multi-pass optical amplifier system comprising:
an amplifier medium;
at least one relay imaging telescope for imaging light from the amplifier medium onto a primary light directing optical component and for imaging light directed back from a primary light directing optical component into the amplifier medium so that light is re-passed through the amplifier medium, and
a phase conjugate mirror arrangement for intercepting light between passes of the amplifier medium to generate a phase conjugate reflection of the light incident on it.
The use of phase conjugation in combination with relay imaging enables high gain operation with significant levels of energy extraction from the amplifier medium. The phase conjugate mirror is preferably located so that light enters it after having passed through the amplifier medium some predetermined number of times. It produces a beam which is conjugate in phase to the beam incident on it, and this conjugate beam retraces the path of the incident beam back through the system to the input point. At this point it has high energy and a spatial phase which is conjugate to the phase of the input beam, so that the high energy beam is of good optical quality. For a phase conjugate mirror, such as a stimulated Brouillon scattering (SBS) cell, which has a threshold power for the input light below which no conjugate beam is generated, the phase conjugate mirror will act as a shutter or an isolator. Thus, gain reduction due to the effects of low power amplified stimulated emissions (ASEs) generated within the system which have an amplitude below the threshold of the phase conjugate mirror is prevented. For pulsed operation of the system the phase conjugate mirror acts as a shutter that only opens when the input beam arrives. This is because the threshold of the phase conjugate mirror is only exceeded when the light pulse is incident on it. In the case of a continuous wave (cw) input beam, the phase conjugate mirror acts as a spectral filter, so that only light in the same spectral bandwidth as the input beam is reflected and so as such it also acts as an inhibitor of ASEs. This is ause only light in the spectral bandwidth of the input beam will exceed the threshold of the phase conjugate mirror.
Preferably, the system is arranged such that a light beam from the amplifier medium which is re-imaged onto the amplifier medium is spatially separated from itself, ie. it does not overlap itself, at at least one point in its path between passes of the amplifier medium to enable it to be intercepted by at least one additional light directing optical component.
The present invention also provides a multi-pass optical amplifier system comprising an amplifier medium and at least one relay imaging telescope for imaging light from the amplifier medium onto a primary light directing optical component and for imaging light directed back from the primary light directing optical component into the amplifier medium, wherein the system is arranged such that a light beam from the amplifier medium which is re-imaged onto the amplifier medium is spatially separated from; itself, ie. it does not overlap itself, at at least one point in its path between passes of the amplifier medium to enable it to be intercepted by at least one additional light directing optical component. This arrangement may include a phase conjugate mirror arrangement as described above for intercepting light between passes of the amplifier medium to generate a phase conjugate reflection of the light incident on it.
If a light beam passes through an aperture at some unknown angle, then it may fail to pass through a second aperture some distance away from the first. This problem can be solved by using a relay imaging telescope, which generally consists of two converging lenses separated by the sum of their focal lengths to image the first aperture onto the second aperture. Then light incident on said first aperture will pass through the second aperture for a relatively wide range of angles of incidence of the beam on the first aperture. Using relay imaging telescopes enables the optical components in the amplifier system according to the present invention to be aligned easily. By spatially separating a light beam travelling through the amplifier system from itself, ie. by causing the light beam not to overlap itself, at some point between passes of the amplifier medium, different parts of the light beam can by intercepted by additional optical components in order to re-direct the light beam through the amplifier medium many times or to remove a beam from the system. Thus, the beam goes through the amplifier many times using one set of optical components. The use of relay imaging telescopes ensures that the light beam stays as an approximately collimated beam as it does so and that it repeatedly goes through the same amplifier aperture. Each time it goes through the amplifier, the light beam experiences gain, and repeatedly extracts energy (or power in the case of continuous lasers) from the same volume of gain material.
The light beam is spatially separated from itself by causing the primary light directing optical component to re-direct the light beam back to the amplifier medium along a different path from the path on which the light beam was incident on the component, for example along a path at a different angle to the optical axis of the system from the incident path or along a path which is parallel to but laterally spaced (with respect to the optical axis of the system) from the incident path. Use of the relay imaging telescopes makes the system according to the present invention xe2x80x9cself imagingxe2x80x9d in that the aperture of the amplifier is imaged onto the primary light directing optical component by a relay imaging telescope, and this component directs the light back through the same telescope so that it is imaged back on to the aperture of the amplifier, so that any collimated beam which goes through the aperture once will immediately go through it again unless intercepted by specially placed additional optical components. In this way light travelling along different paths between passes of the amplifier medium which become spatially separated from each other can be re-imaged onto the aperture of the amplifier medium by a relay imaging telescope. This self imaging property also makes the system according to the present invention relatively simple to align because small errors in alignment do not lead to the loss of light from the system.
The geometry of the amplifier system according to the present invention makes it possible to cause the light path of a light beam passing through the system to overlap at the amplifier medium and spatially separate from itself at at least one part of its path between passes of the amplifier medium, for example, the telescope lenses. In this way each pass of the light beam through the amplifier medium can travel through a large volume of the amplifier medium in order to promote high gain and energy extraction on each pass. Furthermore, the spatial separation of the light beam from itself can allow light to be tapped out of the system or to be re-directed through the amplifier medium along different paths to increase the number of passes through the amplifier medium that can be achieved. The spatial separation of the light beam from itself can also prevent parasitic oscillations within the amplifier system.
In a first preferred embodiment of the present invention a first relay imaging telescope is located adjacent to a first side of the amplifier medium for imaging an aperture of the amplifier medium onto at least one primary light directing optical component located to said first side of said amplifier medium and a second relay imaging telescope is located to a second side of the amplifier medium for imaging the aperture of the amplifier medium onto at least one primary light directing optical component located to said second side of the amplifier medium. Thus, the amplifier medium is located between two relay imaging telescopes which each image the aperture of the amplifier medium onto various appropriately positioned primary light directing optical components which repeatedly pass a light beam through the amplifier medium.
In a second preferred embodiment the amplifier medium itself comprises a reflecting surface located at a first side of said amplifier medium and a relay imaging telescope is located to a second side of said amplifier medium for imaging an aperture of the amplifier medium onto at least one primary light directing optical component located to said second side of said amplifier medium.
The amplifier system according to the present invention preferably additionally comprises input and output optical components which intercept the light beam where it is spatially separated from itself and co-operate with the primary light directing optical component(s) to direct a light beam from an input of the amplifier system, repeatedly through the amplifier medium and then to an output of the system. To improve the ease of optical alignment of the system it is preferred that the input and output optical components are arranged such that the relay imaging telescope(s) images an aperture of the amplifier medium onto the input and output optical components.
The amplifier system is preferably arranged with a geometry such that a light beam passing through the system overlaps its light path in the amplifier medium as it passes repeatedly through the amplifier medium in order to improve gain and energy extraction on each pass of the amplifier as such overlapping enables the same volume of the amplifier medium to be traversed by a light beam passing through the system on each pass. It is further preferred that the light beam does not overlap at the input and output of the system in order to improve the ease with which a light beam can be directed into and out of the system.
Preferably, a light beam travelling along a path from the amplifier medium via the relay imaging telescope and then back to the amplifier medium is spatially separated from itself at or near a focal plane of the relay imaging telescope. This can enable telescope lenses of relatively small focal lengths to be used in the amplifier system according to the present invention.
It is preferred that at least one of the primary light directing optical components comprises a mirror or a prism.
It is also preferred that at least one of the input and/or output optical components comprises a mirror or a prism.
Where phase conjugation is used, the system can additionally comprises a Faraday isolator arrangement for isolating a light beam input into the system from a light beam output from the system.
Phase distortion can be compensated for by making at least one of the light directing optical components an adaptive mirror. In this way it is possible to compensate for phase distortion for each return path through the amplifier medium.
In at least one embodiment, in order to stop parasitic oscillations within the amplifier system before they become dominant, it is preferred that a shutter means is introduced into a part of the light path of a light beam passing through the system which part passes between two light directing optical components, which shutter means can be closed to block the passage of light between the two light directing components at predetermined intervals.
It is possible to produce a high power output beam with a controlled phase front by controlling the phase front of a lower power input beam by adding to the system a beam steering or phase controlling means adjacent to the input of the system and possibly at least one further relay imaging telescopes wherein the system is arranged such that the aperture of the beam steering means is imaged via the further relay imaging telescopes onto an aperture of the amplifier medium.
The amplifier system according to the present invention can be extended to a system comprising two or more amplifying mediums, by coupling each amplifying medium to another, or the other, by a relay imaging telescope. This can generate further increases in the gain of the amplifier system.
The relay imaging telescopes used in the present invention may have a transfer matrix of the type       P    T    =      [                                                      -              1                        ⁢                          /                        ⁢            M                                    0                                      0                                      -            M                                ]  
where M is a positive or a negative number.
It is possible that a relay imaging telescope used in the amplifier system according to the present invention can be made of compound lenses.
According to a second aspect of the present invention there is provided a method of amplifying a beam of light by repeatedly passing said beam through an amplifier medium using a primary light directing optical component by imaging light from the amplifying medium onto said primary light directing optical component and imaging light directed back from a primary light directing optical component back into the amplifier medium using at least one relay imaging telescope and intercepting light between passes of the amplifier medium and using a phase conjugate mirror to generate a phase conjugate reflection of the light incident on it.
This second aspect of the present invention has the same preferred features and exhibits the same advantages as the first aspect of the present invention described above.
There is also provided a method of amplifying a beam of light by repeatedly passing said beam through an amplifier medium using a primary light directing optical component by imaging light from the amplifying medium onto said primary light directing optical component using at least one relay imaging telescope and imaging light directed back from the primary light directing component back into the amplifier medium wherein a light beam from the amplifier medium which is re-imaged into the amplifier medium is spatially separated from itself at at least one point in its path between passes of the amplifier medium so that it can be intercepted by additional light directing optical components.